diff --git a/lib/__tests__/quote-collapse.test.ts b/lib/__tests__/quote-collapse.test.ts
index 3ca03e596..fb7c18591 100644
--- a/lib/__tests__/quote-collapse.test.ts
+++ b/lib/__tests__/quote-collapse.test.ts
@@ -1,4 +1,4 @@
-import { describe, it, expect, beforeEach } from 'vitest';
+import { describe, it, expect, beforeEach, vi } from 'vitest';
 
 import { setupQuoteCollapse, collapsePlainTextQuotes, QUOTE_TOGGLE_ATTR, QUOTE_COLLAPSED_ATTR } from '@/lib/quote-collapse';
 import { plainTextToSafeHtml, sanitizeEmailHtml, sanitizePlainTextRenderedHtml } from '@/lib/email-sanitization';
@@ -162,6 +162,23 @@ describe('setupQuoteCollapse', () => {
     expect(toggle()).toBeNull();
   });
 
+  it('looks for an attribution in one pass over many bare blockquotes', () => {
+    // Every outermost blockquote used to walk all text before it again, so a
+    // body of a few ten thousand blockquotes froze the reader for minutes.
+    const n = 2000;
+    document.body.innerHTML = '<p>Intro</p>' + '<blockquote>x</blockquote>'.repeat(n) + '<p>Earlier:</p><blockquote>late quote</blockquote>';
+    const nodes = document.body.querySelectorAll('*').length + n + 3;
+    const walkerSpy = vi.spyOn(Node.prototype, 'compareDocumentPosition');
+    try {
+      expect(setupQuoteCollapse(document, labels)).toBe(true);
+      expect(walkerSpy.mock.calls.length).toBeLessThan(nodes * 6);
+    } finally {
+      walkerSpy.mockRestore();
+    }
+    expect(document.body.lastElementChild!.textContent).toBe('late quote');
+    expect((document.body.lastElementChild as HTMLElement).style.display).toBe('none');
+  });
+
   it('collapses a nested wrapper case: separator inside a wrapper div hides content outside it too', () => {
     document.body.innerHTML = `
       <div>answer</div>
diff --git a/lib/quote-collapse.ts b/lib/quote-collapse.ts
index 1eab296cb..964ba62b8 100644
--- a/lib/quote-collapse.ts
+++ b/lib/quote-collapse.ts
@@ -86,25 +86,22 @@ function hasVisibleContent(body: HTMLElement, marker: Element, side: 'before' |
  * ending with a colon ("On …, X wrote:", "Am … schrieb X:", "… a écrit :").
  */
 function findAttributedBlockquote(body: HTMLElement): Element | null {
-  for (const bq of Array.from(body.querySelectorAll('blockquote'))) {
-    if (bq.parentElement?.closest('blockquote')) continue; // outermost only
-    if (precedingTextEndsWithColon(body, bq)) return bq;
-  }
-  return null;
-}
-
-/** True when the last non-blank text node before `el` ends with a colon. */
-function precedingTextEndsWithColon(body: HTMLElement, el: Element): boolean {
-  const walker = body.ownerDocument.createTreeWalker(body, NodeFilter.SHOW_TEXT);
+  // One walk in document order, remembering the last non-blank text: a
+  // blockquote is reached before its own text, so that is the text preceding
+  // it. Walking from the start again per blockquote cost quadratic time.
+  const walker = body.ownerDocument.createTreeWalker(body, NodeFilter.SHOW_TEXT | NodeFilter.SHOW_ELEMENT);
   let last: string | null = null;
   for (let node = walker.nextNode(); node; node = walker.nextNode()) {
-    const rel = el.compareDocumentPosition(node);
-    if (rel & Node.DOCUMENT_POSITION_CONTAINED_BY) continue;
-    if (!(rel & Node.DOCUMENT_POSITION_PRECEDING)) break; // reached/passed el
-    const text = (node.nodeValue || '').trim();
-    if (text !== '') last = text;
+    if (node.nodeType === Node.TEXT_NODE) {
+      const text = (node.nodeValue || '').trim();
+      if (text !== '') last = text;
+      continue;
+    }
+    const el = node as Element;
+    if (el.tagName !== 'BLOCKQUOTE' || el.parentElement?.closest('blockquote')) continue; // outermost only
+    if (last !== null && last.endsWith(':')) return el;
   }
-  return last !== null && last.endsWith(':');
+  return null;
 }
 
 /** Hide an element, remembering its original inline display for restore. */